This invention relates generally to an electrophotographic printing machine, and more particularly concerns a screen for extending the range thereof.
In the process of electrophotographic printing, a photoconductive member is charged to a substantially uniform level. A light image of an original document irradiates the charged portion of the photoconductive member dissipating selectively the charge thereon in accordance with the intensity thereof. In this manner, an electrostatic latent image is recorded on the photoconductive member corresponding to the original document being reproduced. The electrostatic latent image is then developed with heat settable toner particles. The toner particles are then transferred from the latent image to a sheet of support material, in image configuration. Heat is then applied to the particles permanently affixing them to the sheet of support material.
Multi-color electrophotographic printing employs a plurality of cycles. Each cycle reproduces a different color contained in the original document. This requires that the light image of the original document be filtered to record an electrostatic latent image corresponding to a single color of the original document. The latent images are developed with appropriately colored toner particles. The toner particles are then transferred to the sheet of support material, in superimposed registration with one another. In this way, a multi-layered toner powdered image is formed on the sheet of support material. This toner powder image is permanently affixed to the sheet of support material by the application of heat to produce a permanent color copy of the original document.
In the past, it has been difficult to produce copies having subtle variations of tone or color. Hence, the reproduction of color slides as opaque copies having pictorial quality has been difficult. In order to overcome this problem, a half tone screen is frequently interposed in the optical light path. This screen produces tone gradations by forming half-tone dots or lines of varying size. In the highlight zones, the dots are small and increase in size through the intermediate shades until they merge together in the shadow regions. At the highlight ends of the tone scale there will be complete whiteness, while at the shadow end there will be nearly solid blackness. Numerous patents describe the concept of screening. Exemplary of these patents are U.S. Pat. Nos. 2,498,732; 3,535,036; 3,121,010; 3,193,381; 3,776,633; and 3,809,555.
In addition to the generally available commercial copying machines arranged to reproduce opaque original documents as opaque copies, many types of printing machines are in wide use for reproducing microfilm. For example, U.S. Pat. Nos. 3,424,525; 3,542,468; and 3,547,533 describe typical microfilm copying machines. Once again, in microfilm copying machines as well as in opaque copying machines, it has been extremely difficult to form copies of transparencies wherein the copy will have pictorial quality. This problem has become more paramount with the advent of multi-color electrophotographic printing machines. In a multi-color electrophotographic printing machine, it is highly desirable to have the capability of reproducing color transparencies, such as 35 mm slides. However, it is required that the copy produced therefrom be of pictorial quality. This requires that a half-tone screen be employed. One type of system employing a half-tone screen for the reproduction of color transparencies is described in copending U.S. application Ser. No. 540,617 filed in 1975. As described therein, a light image of the color transparencies is projected through a half-tone screen disposed on the platen of the electrophotographic printing machine. Normally, the light being projected through the screen is not visible to the eye of the machine operator. In order to determine the focus or lack thereof, the operator will frequently position a sheet of white paper on the screen so as to see the light image being projected through the screen.
One type of screen taught by the prior art is a compound screen. This screen is described in U.S. Pat. No. 3,905,822 issued to Marks in 1975. The screen includes a clear transparent base member having a mixed dot pattern of light absorbing and light reflecting dots.
It is a primary object of the present invention to improve the screen employed in an electrophotographic printing machine reproducing transparencies by having the light image of the transparency visible on the surface of the screen member.